This invention relates to light modulators and more particularly to electro-optical crystal modulators wherein the output light intensity is maintained constant at a given value about which the modulation takes place. This invention is particularly directed to a new control system for an electro-optic light modulator which is adapted to correct thermal effects causing variations in the output light intensity.
It is conventional to employ electro-optic crystals for modulation of coherent laser radiation in, for example, pulse modulation communications systems. In conventional apparatus, a beam of coherent light from a laser is passed through a polarizer to polarize the light in a first plane. The polarized light is passed through an electro-optic crystal modulator which changes the instantaneous orientation of the plane of polarization of the beam in response to an electric field impressed on the electro-optic crystal. The light transmitted by the electro-optic crystal modulator impinges on an analyzer and is transmitted by the analyzer to an extent which is a function of the angle formed by the polarization direction of the analyzer and the instantaneous polarization direction of the laser beam impinging on the analyzer.
A serious problem occasioned by the use of electro-optic crystals as modulators is the variation in light output intensity caused by the change in birefringence of the crystal as a function of temperature. Heretofore, this problem has lead to various types of oven and control systems to alleviate this problem. One system relies on the sensing of a change of temperature at the crystal mount which is fed back as a correction signal to the heater assembly associated with the crystal mount. However, since these temperature sensing devices cannot sense the crystal temperature, but only some temperature at a point in close proximity, a continual drifting of the birefringence and a resulting variation in the depth of modulation of the transmitted light beam is experienced. Further, even if the crystal temperature were monitored accurately, a problem arises from the fact that it is the temperature in a small center portion of the crystal along the optical path which is important. For example, in lasers of the CO.sub.2 type, the heating effect of the laser beam itself can produce large temperature differentials along the cross-section of the electro-optic crystal. Accordingly, sensing the temperature of the outside surface of the crystal is of doubtful value. In addition to the changes of birefringence of the crystal modulator caused by the crystal temperature variations, changes in temperature of any other component which is part of the modulator assembly may also produce changes in the intensity of the output light beam.
Other systems have been proposed to correct for thermal affects in electro-optic modulators by controlling the electric field bias applied to the modulator in response to a feedback signal. These systems utilize a low-frequency probe signal which is superimposed on the modulator bias and is detected by, for example, a photodetector at the output of the laser modulator in order to develop an error signal which is a function of the deviation of the output from a maximum, or other desired optimum output condition. The error signal is used to change the electric field bias applied to the modulator in order to restore the output to the maximum or desired optimum condition. These systems are sometimes adequate for low output level lasers and low modulation frequencies, as the temperature changes encountered in these electro-optic crystal modulator applications are relatively small. However, in the case of a high power laser such as, for example, a CO.sub.2 laser, the modulator absorbs a substantial amount of heat from the laser beam itself. Additionally, at high modulation frequencies the modulating RF energy is also absorbed into the crystal material. This results in substantial changes in temperature which vary according to the type of modulation applied to the crystal modulator. Accordingly, the birefringence of the crystals and therefore the optical path length difference may change over many wavelengths. The voltages required to compensate for these changes ordinarily run into several tens of thousands of volts, which may be well above the dielectric strength of the electro-optic crystal material thereby resulting in a breakdown of the crystal material.
The present invention overcomes the disadvantages of the prior art and proposes a system which further simplifies and reduces the cost of that disclosed in U.S. Pat. No. 3,780,296 issued to the present inventor on Dec. 18, 1973, and assigned to RCA Limited. The present system senses the average intensity of both the input and output beams. The ratio of the intensities is compared with an adjustable external component and the result is used to change the temperature of the modulation crystal or of an external birefringent crystal compensator or external phase shifting device.
The present arrangement is particularly suitable when the following conditions are met:
1. A high carrier-to-noise ratio is always available at the receiving end. In this condition, a slight departure from the optimum conditions is rather inconsequential. This applies particularly to very short communication links.
2. Occasional infrequent readjustments of optical bias are acceptable. Such a frequency would be at the rate of not more than once or twice a day.
3. The system is expected to be at an accessible location to allow the adjustments.
4. The ultimate performance in carrier/noise is not required.
5. The cost must be relatively low and in particular lower than the system disclosed in the U.S. Pat. No. 3,780,296.